An information recording medium on which information can be only once recorded by means of a laser beam is known as a recordable compact disc (i.e., CD-R), and has been getting widely used because it can be produced on a relatively small scale and at low cost, as compared with a conventional compact disc (i.e., CD).
The optical disc of CD-R type generally has a multi-layer structure typically comprising a disc-shaped transparent substrate (support), a recording layer comprising an organic dye, a light-reflecting layer comprising a metal, and a protective layer made of resin overlaid in this order. The information can be recorded by irradiating the disc with a laser beam of near infrared region (usually 780 nm). By the irradiation of the laser beam, irradiated area of the recording layer is locally heated to change its physical or chemical characteristics, and thus pits are formed in the recording layer. Since the optical characteristics of the formed pits are different from those of the area having been not irradiated, the information is optically recorded. The recorded information can be read by a reproducing procedure comprising the steps of irradiating the recording layer with a laser beam having the same wavelength as that employed in the recording procedure, and detecting the light-reflection difference between the pits and the area having been not irradiated.
In order to reproduce the recorded information by means of a commercially available CD player, the CD-R is required to reflect the laser beam of near infrared region at a reflectance of more than 70%. Therefore, the light-reflecting layer is preferably provided on the recording dye layer. The light-reflecting layer is generally made of Au (gold), because it gives a high reflectance and a high corrosion resistance. However, the light-reflecting layer made of gold has certain disadvantageous features in that gold is expensive and further that the protective layer provided on the light-reflecting layer of Au (gold) exhibits golden gloss which makes printed letters or figures on the protective layer unclear.
The light-reflecting layer can be made of metal other than gold. Japanese Patent Provisional Publications No. H3-286432 and No. H4-49539 disclose an optical information recording medium having a light-reflecting layer made of Ag (silver), Cu (copper) or alloys essentially consisting of them. According to Japanese Patent Provisional Publication No. H3-286432, the light-reflecting layer of Ag (silver) or Cu (copper) has a reflectance higher than that of the layer of Au.
As the dye compound contained in the recording layer, a cyanine dye having a benzoindolenine structure (in which a benzene ring is condensed with an indolenine skeleton) is stated to be advantageously used (Japanese Patent Provisional Publication No. H4-175188), and further the cyanine dye having ClO.sub.4.sup.- ion is particularly preferred to use.
However, the present inventors have found that the cyanine dye having ClO.sub.4.sup.- ion in the recording dye layer is liable to lower the reflectance of the light-reflecting layer of Ag or Ag-containing alloy.
Accordingly, it is an object of the present invention to provide an optical information recording medium having a high light-reflectance. Particularly, the object of the invention is to provide an optical information recording medium having a high light-reflectance in the case that the light-reflecting layer is made of Ag or Ag-containing alloy.